Heat-recoverable articles are articles the dimensional configuration of which may be made substantially to change when subjected to heat treatment.
Usually these articles recover, on heating, towards an original shape from which they have previously been deformed but the term "heat-recoverable", as used herein, also includes an article which, on heating, adopts a new configuration, even if it has not been previously deformed.
In their most common form, such articles comprise a heat-shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Pat. Nos. 2,027,962; 3,086,242 and 3,597,372. As is made clear in, for example, U.S. Pat. No. 2,027,962, the original dimensionally heat-stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded, whilst hot, to a dimensionally heat-unstable form but, in other applications, a preformed dimensionally heat-stable article is deformed to a dimensionally heat-unstable form in a separate stage.
In the production of heat-recoverable articles, the polymeric material may be cross-linked at any stage in the production of the article that will enhance the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises shaping the polymeric material into the desired heat-stable form, subsequently cross-linking the polymeric material, heating the article to a temperature above the crystalline melting point or, for amorphous materials the softening point, as the case may be, of the polymer, deforming the article and cooling the article whilst in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape.
Heat-recoverable articles have become widely used for forming solder connections between electrical conductors in view of the ease of forming the connection and the quality of the connection so formed.
For such applications the article, usually in the form of a sleeve, contains a quantity of solder for forming the electrical connection and a pair of fusible inserts for sealing the connection. These articles are described for example in U.S. Pat. Nos. 3,243,211, 4,282,396 and 4,283,596, the disclosures of which are incorporated herein by reference, and are sold by Raychem Corporation, Menlo Park, Calif. under the trade mark "SOLDER SLEEVE".
Such heat-recoverable articles may be used for forming a stub splice, between a pair of wires, in which case both wires are inserted into one end of the sleeve, and the sleeve is then recovered. The devices are usually manufactured with a fusible polymeric sealing ring in the region of each end of the sleeve to provide a seal when the sleeve is recovered about the conductors. Some devices, for example, as described in European Patent Application No. 159,945 are provided with a cylindrical insert in one end of the device which will act as a seal. We have found, however, that the articles employed in the prior art suffer from a number of disadvantages. For example, when the article contains a ring-shaped seal, the seal must be completely melted during recovery of the article so that it will act as a plug. Such a degree of heating is not only time consuming when large quantities of articles are to be installed but also runs risk that the article will be overheated which may cause the article itself or electrical insulation of the conductors to be damaged, or may cause the formation of a poor joint due to migration of the solder. In addition, when the insert is fused it will tend to flow along the sleeve, both towards the centre of the sleeve and out of the end of the sleeve. This fusion of the insert prevents the insert from being used reliably as a stop to limit insertion of the conductors, and also may necessitate lengthening of the sleeve in order to prevent fused sealant flowing out of the end of the sleeve. When a cylindrical insert is employed considerable problems can be experienced during manufacture due to mis-orientation of the insert within the sleeve which can cause very high rates of scrap during production of the articles. In order to reduce scrap rates it is possible to overheat the article during installation about the wires so that the insert is completely melted and the irregular profile of the sleeve disappears. However, such a solution leads to the same problems of overheating as described above.